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A system-wide ground stop was issued for Alaska Airlines and Horizon Air flights at the Seattle-Tacoma International Airport (Sea-Tac) Sunday night after an IT outage disrupted operations nationwide.

The IT outage began around 8 p.m., leading the airline to temporarily ground all flights operated by Alaska and its regional partner Horizon Air. The ground stop lasted for approximately three hours.

Sea-Tac is reporting 39 flight delays and 45 cancellations in response to the three-hour ground stop. Since Sunday evening, Alaska Airlines has had more than 150 flight cancellations overall.

“As we reposition our aircraft and crews, there will most likely be residual impacts to our flights,” Alaska Airlines wrote on X. “It will take some time to get our overall operations back to normal. We apologize for the inconvenience and encourage guests to check the status of your flight before leaving for the airport.”

At approximately 8 p.m. Pacific on July 20, we experienced an IT outage that resulted in a temporary, system-wide ground stop for Alaska Airlines and Horizon Air flights.

As of 11 p.m. Pacific, the ground stop has been lifted, and our operations have resumed.

As we reposition…

— Alaska Airlines News (@AlaskaAirNews)

Alaska Airlines cited that an issue with third-party hardware at a data center caused the ground stop, not a cybersecurity issue.

“A critical piece of multi-redundant hardware at our data centers, manufactured by a third-party, experienced an unexpected failure,” Alaska Airlines stated Monday. “When that happened, it impacted several of our key systems that enable us to run various operations, necessitating the implementation of a ground stop to keep aircraft in position. The safety of our flights was never compromised.”

Anyone taking an Alaska Airlines or Horizon Air flight on Monday should check their flight status before heading to the airport, and to expect potential delays.

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